At least 36 people were feared dead after a fire raged through a psychiatric hospital north of Moscow today, killing some patients in their beds and others who were trapped by barred windows.

The fire broke out in the early hours and swept through a single-storey building at the hospital in woods in the village of Ramensky, officials said. Twelve bodies had been found by mid-morning, the Emergencies Ministry said, as grey smoke billowed from the charred yellow-brick building.

Andrei Vorobyov, interim governor of the Moscow region, said a nurse had led two patients to safety. He said 36 others were believed to have been in the hospital when the fire broke out, but another local official said 38 were feared dead. “Those who were in there said it happened in a flash. The nurse opened the door to the room and there was smoke, and even when she saw the fire she could not get to the fire extinguisher. It all happened very quickly,” he told Russia 24 television.

He said some windows had been barred to meet regulations while others had not, so the investigators would be able to determine whether they had prevented people from escaping. “Obviously, all the patients were sleeping and they were sick people ... so they would have needed help to get out,” he said, adding that the nearest fire station was a 40-minute drive away.

President Vladimir Putin called for an explanation of the “tragedy” and told emergency services to do all they could to help. Fires at state institutions in Russia such as hospitals, schools, drug treatment centres and homes for the elderly or disabled often cause casualties, raising questions about safety measures, conditions and escape routes.

Dmitry Pestov, deputy head of the Moscow region, said 41 people had been in the building but three had been saved and were in hospital. “ Fire safety watchdogs constantly check all (public) institutions and issue recommendations. As far as I know, all the recommendations had been followed,” he told reporters outside the hospital which still smouldered.

“There are different versions, including arson and a short circuit. They will be checked.” Some people stood on the opposite bank of the Moscow canal from the hospital, trying to get across to check whether their relatives had survived. The police had stopped the ferry and fishing boats were not allowed to cross.

“They are not letting the relatives in. Why? How can we get there?” said Konstantin, whose father was in the hospital, which is about 120 km north of Moscow. “Living conditions? It was a slum in there. No conditions.” More than 12,000 people were killed in fires in 2011 and more than 7,700 in the first nine months of 2012 in Russia, where the per capita death rate from fires is much higher than in Western nations including the United States.